The object of class ostream that represents the standard logging stream oriented to narrow characters (of type char). It corresponds, along with cerr, to the C stream stderr. The standard logging stream is a destination of characters determined by the environment. This destination may be shared by more than one standard object (such as cout or cerr).
Object of class ostream, characters can be written to it either as formatted data using the insertion operator (operator<<) or as unformatted data, using member functions such as write.
Following is the declaration for std::clog.
extern ostream clog;
By default, clog is synchronized with stderr.
A program should not mix output operations on clog with output operations on wclog or wcerr (or with other wide-oriented output operations on stderr): Once an output operation has been performed on either, the standard logging stream acquires an orientation (either narrow or wide) that can only be safely changed by calling freopen on stderr.